1. Field of the Invention
The present invention relates to a mobile radio communication system, and more particularly to a reserved channel allocating method in a mobile radio communication system which can adaptively control the number of reserved channels in accordance with the fluctuant handoff traffic density rapidly changing in a specified geographic service area.
2. Description of the Related Art
Generally, a base station in a mobile radio communication system determines the number of normal channels and reserved channels by setting a plurality of traffic channel elements. Here, the normal channels are allocated for the process of initial calls and calls caused by handoff (hereinafter, referred to as handoff calls), and the reserved channels are allocated only for the process of the handoff calls only. Specifically, if a call is initiated from a certain mobile unit, it is processed through the normal channel. If a handoff call is initiated, it is first processed through the normal channel, but if all the normal channels are in use at that time, it is processed through the reserved channel. Generally, the number of normal channels is determined to be about 70% of the whole number of the channels, and the number of reserved channels is determined to be about 30% of the whole number of the channels. Here, the term "handoff" defines a function whereby if the mobile unit crosses the boundary between two geographic service areas, i.e., two cells, it is first allocated two channels from base stations of the two cells, and then if the intensity of the signal received from the previous base station becomes weak, it is automatically tuned to the channel of the current base station.
Since the mobile radio communication system provides such a handoff function, the number of calls attempted by mobile units, as the mobile units freely move across the boundaries among the cells, should have been greatly increased. Especially, during the time period for workers' going to work or going home from work, the traffic intensity caused by such handoffs in a particular downtown area may be greatly increased in comparison with other areas.
Specifically, in such downtown areas, a number of mobile units may be frequently allocated reserved channels from the corresponding base stations to process the handoff calls during almost the same time period.
However, since the number of normal channels and the number of reserved channels have already been set at a fixed rate, the normal channels for processing the initial calls may be changed to the reserved channels for processing the handoff calls in the area where a number of handoff calls are initiated during almost the same time period as described above, causing the telephone services not to be achieved. In other words, if the handoff call is initiated, it is first processed through the normal channel, but if all the normal channels are in use at that time, it is processed through the reserved channel.
For instance, if the number of initial calls is continuously increased in a state that about 20% of the whole channels are allocated for the initial calls and about 50% of the whole channels are allocated for the handoff calls in a particular service area, the telephone services cannot be provided with respect to any further initial call since the remaining 30% of the whole channels have already been allocated as the reserved channels and thus they are not available.